Sustained solar fuel production

Bishnu P. Biswal and Bettina V. Lotsch

One of the grand challenges facing today’s society is to replace fossil fuels by sustainable and clean energy sources. In this context, the conversion and storage of solar energy in the form of chemical bonds in “solar fuels” such as H₂ through light-driven H₂O reduction has evolved into a key technology over the last decade, driven by fast depletion of fossil energy sources and rapid global climate change. Inspired by natural photosynthesis the major challenge is to find a robust and highly active, yet low-cost and earth-abundant catalytic system in combination with a suitable heterogeneous photosensitizer.

The research into solar H₂ evolution from water utilizing covalent organic frameworks (COFs) as a new family of carbon-based heterogeneous photosensitizers has gathered significant momentum recently. This is due to a number of attractive features of COFs, including their crystallinity and ordered porosity, controllable light harvesting and photophysical properties, as well as their molecular definition and tunability. However, until now, there are only very few reports in which COFs are utilized as photoabsorber for photocatalytic H₂ evolution, mostly with metallic platinum (Pt) as the co-catalyst (a rare and expensive metal), which appears to be the bottleneck towards scalable, economical solar H₂ production. In addition, the use of nanoparticulate Pt co-catalysts precludes detailed insights into the nature of the catalytic sites and the intricacies of the photocatalytic cycle.

A research group led by Prof. Bettina V. Lotsch, Nanochemistry Department at the Max Planck Institute for Solid State Research in Stuttgart and Cluster of Excellence e-conversion, in collaboration with the Theoretical Chemistry group led by Prof. Christian Ochsenfeld at LMU Munich and e-conversion, has now developed a single-site thiazolo[5,4-d]thiazole-linked COF (TpDTz) based photocatalytic system, which is fully earth-abundant and based on a noble-metal-free nickel-thiolate hexameric cluster co-catalyst (NiME) with well-defined catalytic centers unlike metallic Pt. This system is the first COF photocatalyst that is photochemically very stable and operates with a noble-metal free co-catalyst in water over 70 hours together with triethanolamine (TEoA) as the sacrificial electron donor. The excellent H₂ evolution rate and long-term photocatalytic operation of this hybrid system in water surpasses that of many known state-of-the art organic dyes, carbon nitride and COF-sensitized photocatalytic water reduction photocatalytic systems. In addition, this study puts forward a unique photocatalytic reactor design which enables the non-invasive and direct monitoring of the H₂ evolution rate with high accuracy, in contrast to the routinely used standard photocatalytic batch reactors. This design allows to gather unique insights into the photocatalytic reaction mechanism and the complex kinetics of the reaction system, which is derived using microkinetic theory.

Overall, this work adds a new dimension to the rational design of robust, nobel-metal free and efficient single-site COF–molecular co-catalyst hybrid systems for the sustained generation of solar fuels from pure water. In the future, the researchers expect to further fine-tune the COF-co-catalyst interactions to boost the performance of such hybrid photocatalytic systems, and expand the scope of solar-to-chemical energy production.